1A. What you will draw
You will draw two motor units side by side. One is small (for fine control, like an extraocular eye muscle). One is large (for power, like the quadriceps). The contrast in your drawing should make the size principle obvious.
BIO 304 . WEEK 4 . MONDAY . LAB WORKBOOK
Motor Units and Muscle Mechanics
Motor unit organization, recruitment, summation, and fatigue.
Print this page. You will draw your own diagrams from the directions below, then hand-label the structures listed. Drawing by hand is the integrity mechanism for this course.
Part 1 of 2
Anatomy Lab
Box A. Two motor units, side by side
Directions
- On the LEFT, draw a small circle for a motor neuron cell body in the ventral horn of the spinal cord. Extend an axon downward.
- Branch the axon into 5 short terminals, each ending on a different muscle fiber. Draw 5 short ovals as the muscle fibers.
- Label this side Small motor unit (eye muscle).
- On the RIGHT, draw another cell body and axon. Branch it into many terminals (draw 12 to 20). Draw the same number of muscle fibers.
- Label this side Large motor unit (quadriceps).
- Add labels: Motor neuron cell body, Axon, Axon terminal, Neuromuscular junction, Muscle fiber.
Draw here. Sketch by hand.
Box B. Twitch summation (force vs time)
Directions
- Draw an x-axis (time) and a y-axis (force).
- On the same axes, sketch three force traces stacked vertically by frequency.
- Trace 1: single twitches at 1 Hz. Force rises and falls completely between each stimulus. Label Single twitches.
- Trace 2: stimulation at 10 Hz. The second twitch starts before the first finishes; force adds up. Label Wave summation.
- Trace 3: stimulation at 30 Hz. Twitches fuse into a smooth, sustained plateau. Label Complete tetanus.
- Below the graph write one sentence: why does higher frequency produce more force?
Draw here. Sketch by hand.
1C. Structures to label (10)
After you finish each drawing, label every structure below directly on your sketch.
- Motor neuron cell body
- Axon
- Axon terminal
- Neuromuscular junction
- Muscle fiber
- Small motor unit
- Large motor unit
- Single twitch
- Wave summation
- Complete tetanus
Part 2 of 2
Physiology Lab
2A. Fiber type comparison table
Fill in the table below. Use one short phrase per cell. After the table, answer the two interpretation questions in complete sentences.
| Property | Type I (slow oxidative) | Type IIa (fast oxidative) | Type IIx (fast glycolytic) |
|---|---|---|---|
| Myosin ATPase rate | |||
| Mitochondria density | |||
| Capillary supply | |||
| Fatigue resistance | |||
| Primary energy system |
Which fiber type would dominate the postural muscles of the back? Justify in one sentence.
A 100-meter sprinter and a marathon runner are tested. Whose calves would have a higher percentage of Type IIx fibers? Whose would have the most mitochondria? Justify each.
2B. Synthesis questions
Answer each in 2 to 4 sentences. Use the language from this week's lecture and your drawings as evidence.
1. Eye muscles are innervated by very small motor units (only a handful of fibers each). Explain why this design works beautifully for precision tracking but would fail for lifting a heavy object.
2. Apply the size principle. A person picks up a coffee cup. Then the same person attempts a deadlift. Which motor units are recruited in each case, and in what order?
3. Train a sprinter on explosive jumping and a marathoner on long slow distance for six months. Predict which fiber type each adapts most strongly and the physiological mechanism behind the adaptation.
3. What to submit
Complete both the Anatomy Lab (your own drawings, hand-labeled, plus the structures list) and the Physiology Lab (activity and synthesis questions). Photograph or scan every page and upload to Canvas before the deadline listed on the schedule. Hand-drawn, hand-labeled work is the integrity mechanism for this course. Typed or AI-generated diagrams are not accepted.